Nitrides offer a unique range of properties compared to other semiconducting materials like silicon and silicon carbide. Wide band gap materials like gallium nitride (GaN), aluminum gallium nitride (AlGaN), and aluminum nitride (AlN) are finding applications in high frequency devices, high power devices, LEDs, optoelectronics, bio-technology, and high efficiency electronics. The opportunity exists for integration of these devices into a single nitride layer. Nitrides offer a unique mix of optical transparency through the visible and UV spectrum, high thermal conductivity, chemical resistance, piezoelectric properties, laseability, biocompatibility and high frequency capability.
The use of nitrides in most applications has been limited by the lack of low cost high crystal quality material. Nitrides have proven difficult to grow economically in high quality single crystal form due to the high temperatures and narrow growth conditions. While this work may eventually yield low defect density material, the cost of those materials will be inherently high.
In most applications, freestanding nitride layers are preferred. Freestanding layers typically yield higher performing devices, the ability to cleave, implant within in the body, form piezoelectric elements, lase high precision parts, regrowth low defect density layers, eliminate extra thermal boundary layers, operate in harsh chemical environments, operate at higher frequencies, form a freestanding waveguide, anneal, and form a 3 dimensional structure. These uses and devices are all compromised if a growth substrate is required.
In most of these cases, the need is for thin (less than 200 micron thick) nitride layers with sufficient area to allow for economical processing. HVPE in particular has been used to grow thick layers typically 1 cm thick on a variety of growth substrates. The thick layer is then sliced into thick wafers typically 400 microns thick. These layers are usually 1 cm×1 cm in area due to difficulty in growing large area wafers using this approach. Several authors have disclosed the use of separation techniques to remove thin layers of nitride from a growth substrate. These techniques include chemical etching of the growth substrate, mechanical grinding, laser separation, and the use of release layers such as void assisted separation. The problem with these approaches is that the stresses induced from the lattice mismatch between the growth substrate and the nitride layer tends to create bowing issues with the nitride layer on the growth substrate and the freestanding nitride layer. In addition, these techniques tend to fracture or damage the nitride layer unless very thick layers are grown. These very thick layers tend to exhibit high degree warpage due to the lattice mismatch between the growth substrate and the nitride layer. Bow on a 100 micron thick HVPE layer grown on 3 inch sapphire wafer can easily exceed several hundred microns across the wafer. This leads to either the need for significant material removal or the use of a supporting substrate.
Waferbonding is typically used to support thin nitride layers which compromises the usefulness and surface properties of the nitride layer. The need exists for techniques, which can create low cost thin nitride layers with sufficient area and flatness to be useful in a wide range of applications. The need also exists for techniques in which the curvature of and/or stresses within thin nitride layers can be controlled. The intent of this invention is to disclose stress control and/or annealing techniques based on radiation processing which meet these needs. The need also exists for packaging, devices, and applications which can take advantage of the material properties these nitride layers provide. The need for freestanding nitride films is driven by a number of requirements. Crystal quality is one important factor.
Nitride crystal quality has been the subject of significant study due to its effect on device performance. Surprisingly, the lack of a cost effective bulk crystal growth method for nitrides has not hindered a variety of applications to be created especially light emitting diodes (LEDs). However, the need still exists for free standing high crystal quality nitride layers for a variety of applications such as high powered devices. While as stated earlier, thick HVPE grown freestanding nitrides have been demonstrated, they are not cost effective and cannot be grown as large areas. Typically 400 to 500 micron thick wafers must be sliced from cm thick boules grown on non-native substrates. These wafers require subsequent epitaxial polish to create an epitaxial ready surface. After processing, the wafers must be thinned to a reasonable thickness if low thermal impedance is required. The limited size, thickness and defects created by the epitaxial polishing limits this approach as a cost effective process.
A variety of techniques have been used to create cost effective nitride epitaxial layers on non-native substrates. These efforts typically have been focused on nitride growth on non-native substrates like sapphire, SiC, silicon, glass and various other materials. In each case, high defect densities in excess of 108/cm2 are typically created. In these cases, the non-native substrate typically degrades the thermal performance of the device or must be removed. Unfortunately, the stresses in nitride layers grown in this manner require the use of support substrates, which then degrade the thermal performance of the device. Various regrowth techniques for reducing the number and type of defects have been demonstrated such as epitaxial lateral overgrowth of a seed or initial nitride layer. These regrowth techniques typically require additional patterning steps, which limits the cost effectiveness of this approach. They also all require the use of a supporting layer which means that the inherent stresses and strains induced between the non-native substrate and the nitride layer during the initial growth are still present even during the regrowth process. The need exists for cost effective techniques, which enable regrowth of higher crystal quality layers on freestanding nitride layers.
This invention creates thin freestanding nitrides layers of size and flatness sufficient for regrowth as well as feature formations along specific crystal planes which can be used reduces defect densities of the subsequent regrowth layers. The ability to grow polar and non-polar nitride layers on non-native substrates using cost effective methods such as HVPE has been demonstrated. The ability of create freestanding thin nitride layers both polar and non-polar with sufficient flatness, area, and mechanical integrity for handling, processing, and regrowth processes is disclosed in this invention. The unsupported nature of the thin nitride layers also reduces defect density in the regrowth layers.
Using the techniques of this invention, a variety of surface features can be created during the separation process. More specifically, very high aspect ratio features not possible with etching techniques can be formed simultaneous to separation. The spacing of these features can be less than 5 microns. Removal/separation techniques can create thin higher crystal quality thin nitride layers after regrowth by removing at least a portion of the original nitride layer. The use of this approach is preferred especially for the purpose of forming higher quality AlGaN and AlN layers from an original GaN layer.
The need also exists for techniques in which the strains induced by epitaxial growth on non-native substrates can be manipulated for improved device performance, packaging, and cooling. Strain control in silicon has proven a very effective tool in increasing mobility of electrons. While this effect is on atomic scale, the control of strain based on the physical constraint of the nitride layer can modify the device performance. This effect can control device performance.
While SiC has a higher thermal conductivity than high dislocation defect density GaN, low dislocation defect density GaN approaches the thermal conductivity of SiC and may eventually be higher than SiC as crystal quality, point defects, and purity improves. More importantly, high power density devices are not limited by just the thermal conductivity of the materials. Typically thermal boundary interface resistances dominate as power densities exceed several W/mm2. While it would appear obvious that the higher thermal conductivity of SiC, diamond, and other exotic materials would enhance the thermal performance of a device, in actuality, there is a fundamental problem associated with any non-native growth substrate. Phonons reflect at epitaxial boundaries just as photons reflect at index changes due to Fresnel reflections. In addition, added thickness is required to compensate for the stresses induced due to thermal expansion and lattice mismatch between the nitrides and the non-native substrate. Since the temperature difference from a purely thermal conduction standpoint is directly proportional to thickness it is important to minimize the distance between the junction and the cooling media. As an example, typically more than 100 microns of SiC is required at 350 W/m/K to support a GaN epitaxial layer and maintain reasonable flatness for die sizes with areas of 1 mm2. Low dislocation GaN has thermal conductivity exceeding 200 W/m/K.
Using the techniques disclosed in this invention, the present inventors have been able to create large area (greater than 1 cm2) flat nitride layers with thicknesses less than 30 microns without any additional support structure. Taking only bulk thermal conduction into account, the junction temperature of the GaN on GaN device will be half that of the GaN on SiC device. If the thermal boundary layer created by the epitaxial mismatch between the GaN and SiC is also included in the model, the temperature difference between the two example die is even more dramatic. There is also experimental data, which shows that the thermal boundary resistance increases with temperature, which can lead to a thermal runaway condition for the GaN on SiC device. Finally, the transient thermal response of the GaN on SiC is even more problematic because the thermal boundary layer is typically in close proximity to the junction, which exhibits very low thermal mass. In a manner similar to an optical cavity, localization can occur limiting significantly the operation point of the GaN on SiC with regard to high power pulsed operation. This localization of heat right at the junction area is mainly due to multiple reflections of the phonons at the GaN/SiC interface. The need exists therefore for nitride substrates, device structures and packaging which minimizes the distance between the junction and the final cooling means and also reduces the number of thermal boundary resistance interfaces such that very high power densities both CW and transient can be realized.
The anisotropic nature of the nitride crystal structure has a significant impact on the performance of the devices made using this material. Significant effort has been put forth on using multiple growth planes to control high current droop and other performance parameters in LEDs and electrical devices. These effects mainly relate to the piezoelectric fields created within the growth layers and how they effect the movement of charge within the device. The mounting configuration and how it mechanically constrains the device, at least for thin nitride layers, can also influence device performance. It appears that the stresses induced by constraining the nitride layer either via a non-native growth layer or via a support layer to which a thin nitride layer is transferred via waferbonding and laser liftoff has a significant effect on the optical, electrical, thermal, and mechanical performance of the resulting device. The need exists for techniques for controlling stress in thin nitride layers and devices and packaging, which takes into account the effect of stresses created by restraining the thin nitride layer. A wide range of devices properties are affected by these issues ranging from droop in LEDs to reduced operating range in HEMTs etc.
Dutta (U.S. Pat. No. 4,456,490) first disclosed in the 80s the use of backside laser irradiation through a substrate substantially transparent to the laser irradiation to modify a semiconductor layer on the substrate, which was absorptive to the laser irradiation. Several other authors have disclosed the use of backside laser irradiation to modify/convert at least a portion of an absorptive semiconductor layer on a substantially transparent substrate. They disclose the use of irradiation in a wavelength range transparent to the substrate but absorbed by the semiconductor layer. In this approach, the decomposition of GaN into gallium and nitrogen creates an explosive reaction at the interface, which leads to separation due to the expansion of the nitrogen and conversion of the GaN is gallium metal.
Alternately, other authors have disclosed the use of various mechanical, thermal, and chemical means to allow for separation of the semiconductor layer from the non-native substrate. Practically, thin semiconductor layers have required bonding techniques to prevent cracking and damage to the semiconductor layer due to the explosive nature of the laser liftoff approach. Bonding is also required to maintain flatness due to the lattice mismatched that exists between the semiconductor layer and the non-native growth substrate.
The formation of localized stress features can be used to control the flatness of thin nitride layers. The spacing, geometry, direction relative to the crystal planes, and depth of these stress features determines the flatness of the layer. In this approach, stress features at typically separated spatially. The formation of stress features on the other side of the semiconducting layer is also disclosed. The localized nature of this approach can be additionally be used to separate semiconductor layers which do not exhibit absorption to the wavelength of the radiation used for separation. In this manner, semiconductor layers such as ALN, AlGaN (with high aluminum content) as well as other high bandgap materials can be separated from a substrate. This also allows for separation of semiconductor layers with nucleation layers transparent to the radiation being used. The localized nature of this approach can take advantage of the non-linear optical properties created at an epitaxial boundary.
Transparent nucleation layers exhibit index of refraction variation, including scatter, Fresnel reflections, non-linear index changes, and/or a combinations of these effects lead to localization of the energy from the radiation source. This technique is therefore not limited to semiconducting layers which exhibit absorption to the irradiating source as disclosed in the prior art. In the case of mechanical means and chemical etching means used to remove the non-native growth substrate, thin semiconductor layers still require bonding layers to maintain flatness due to the stresses created during growth of the layers.
A need exists for the development of techniques to form thin flat freestanding high crystal quality nitride layers such that heat can be removed from the junction as rapidly as possible. Methods to maintain/control flatness in thin semiconductor layer without additional thermal interfaces or support substrates are also needed. Thin semiconductor layers based on stress control features can be formed simultaneous with the separation of the semiconductor layer from its non-native substrate. This technique creates textured surfaces for enhanced optical, enhanced thermal extraction, and/or enhanced regrowth. This invention also discloses methods and articles that relate to using these thin nitride layers by themselves or in conjunction with other materials to form single sided, double sided and three-dimensional circuitry. Microchannels and other cooling means in at least one of the layers can extract heat efficiently. By manipulating the laser intensity profile and cutting pattern not only can optical, thermal cooling and regrowth features by created in the semiconducting layer but also very thin semiconducting layers can be separated without cracking.
These freestanding nitride layers enable a host of the devices ranging from but not limited to solar cells, LEDs, laser diodes, power switches, HEMTs, and other semiconducting devices. Due to the anisotropic nature of the nitride crystal structure, stress within the layer can have significant impact on the devices grown on the layer.
The present inventors have discovered that the spectral distribution of an unsupported LED is significantly different from the spectral distribution of the same die rigidly mounted. The ability to control the stress profiles within large area nitride devices is important to optimizing device performance and operational ranges. The formation of these thin semiconducting layers also eliminates the need for secondary removal of excess thickness via etching and or grinding as is required in the use of bulk nitride wafers. The ability to create epitaxial-ready thin nitride layers which do not required additional polishing steps has also be demonstrated using this technique by the present inventors and is an embodiment of this invention. These thin nitride layers can be used within multilayer packages with and without devices. These thin nitride layers can be used as heat spreaders, submounts, bimorphs, and structural elements within electronic, optical and optoelectronics packages.
In general, thin nitride layers can replace SiC, AlN, sapphire, alumina, beryllium oxide, metal composites, and other thermally conductive low thermal expansion materials due to its high thermal conductivity, low optical absorption, low coefficient of thermal expansion, chemical resistance, laser machinability, piezoelectric properties, non-linear optical properties, cleavability, bio-compatability as well as other physical properties. The overall intent of this invention is to disclose articles and methods that benefit/need the combination of material properties that nitrides provide.